1. Field of the Invention
Embodiments relate to a passive optical network (PON), and more particularly, to an optical line terminal (OLT) and an optical network unit (ONU) included in a PON.
2. Description of the Related Art
A passive optical network (PON) may employ a time division multiple access (TDMA) scheme of providing a service by dividing time, and a wavelength division multiplexing (WDM) scheme of providing a service by dividing a wavelength, based on a transmission scheme.
TDMA-PONs employing the TDMA scheme may be classified into an Ethernet PON (EPON) and a gigabit-capable PON (GPON) based on a used transmission protocol. The EPON and the GPON may transfer signals in a downstream direction based on a broadcast scheme and may transfer signals in an upstream direction based on the TDMA scheme, using a point-to-multipoint link architecture. To allow optical network units (ONUs) to transmit signals in the upstream direction based on the TDMA scheme without a collision, an optical line terminal (OLT) may determine a transmission time of the ONUS based on a band allocation algorithm.
The EPON may include, for example, a 1G-EPON that supports a transmission rate of 1.25 gigabits per second (Gbps) in the downstream direction and the upstream direction, an asymmetric 10G/1G-EPON that supports a transmission rate of 10.3125 Gbps in the downstream direction and that supports a transmission rate of 1.25 Gbps in the upstream direction, and a symmetric 10G/10G-EPON that supports a transmission rate of 10.3125 Gbps in the downstream direction and the upstream direction. The EPON may support a transmission of an Ethernet frame with a length of 64 bytes to 1,518 bytes.
To allocate a transmission band to at least one ONU connected to an OLT in the EPON, the OLT may use a GATE multi-point control protocol data unit (MPCPDU) message with a length of 64 bytes. The OLT may allocate a transmission time to an ONU based on a grant start time field of 4 bytes and a grant length field of 2 bytes included in the GATE MPCPDU message. When a local clock value of an ONU that receives the GATE MPCPDU message is identical to a time of the grant start time field, the ONU may turn on a laser of an optical transceiver and may transmit an Ethernet media access control (MAC) frame to the OLT during a time designated in the grant length field.
The ONU may subtract a time required to transmit the Ethernet MAC frame from a time allocated based on the grant length field. When a length of the time required to transmit the Ethernet MAC frame is less than a length of a time obtained by the subtracting, the ONU may transmit the Ethernet MAC frame. When the length of the time required to transmit the Ethernet MAC frame is greater than the length of the time obtained by the subtracting, the ONU may transmit idle data instead of the Ethernet MAC frame. When the time allocated based on the grant length field elapses, the ONU may turn off the laser and stop transmission.
Currently, the EPON does not provide a fragmentation function of the Ethernet MAC frame. When a length of the time allocated based on the grant length field or the length of the time obtained by the subtracting is less than the length of the time required to transmit the Ethernet MAC frame, the ONU may waste a band by transmitting idle data. A phenomenon in which the ONU wastes a band may become serious as a cycle time of transferring a GATE MPCPDU control message decreases or as a number of ONUs connected to an OLT increases.